The present invention relates to production of polymer resin, and more particularly to a method and an apparatus for controlling a multistage counter current wash train used for the recovery of polymer in a polymerization process.
The production of arylene sulfide polymers by the reaction of at least one polyhalo-substituted cyclic compound with an alkali metal sulfide in a reaction medium comprising a polar organic compound is disclosed in U.S. Pat. No. 3,354,129, issued to Edmonds and Hill. The disclosure of this Edmonds and Hill patent is hereby incorporated by reference.
The poly(arylene) sulfide, hereinafter referred to as PAS, of most industrial importance is a semi-crystalline polyphenylene) sulfide polymer, herein after referred to as PPS, which is produced commercially in a reaction of a sodium sulfide equivalent, e.g., an approximately equal mixture of sodium hydrosulfide (NaSH) and sodium hydroxide (NaOH) followed by the addition of p-dichlorobenzene (DCB) in a reaction medium of a cyclic amide, such as N-methyl-2-pyrrolidone (NMP).
The presently preferred method for recovery of the PPS reaction product involves use of several counter current wash trains. The PPS process stream entering a counter-current washing operation is a solids cake saturated with an original solvent. As used herein, a cake is a discrete mass of solid polymer particles. The main objectives of a washing operation are to displace the original solvent that is wetting the solids cake with the wash solvent, and wash away dissolvable solids that might be contained in the cake. With an extreme amount of washing, e.g., many wash stages with very large wash solvent flow rates, essentially none of the original liquid solvent will remain on the cake exiting the final wash stage, i.e., the liquid on the exiting cake would be entirely the wash solvent. An economically desired degree of washing, however, does not displace all of the original liquid solvent with the wash liquid solvent. Accordingly, a small percentage of the original liquid solvent will remain on the solids cake exiting the wash train.
An installed counter-current wash train has a fixed number of stages, which will generally vary from two to six. In operation of a wash train in a PPS process, it would be highly desirable to maintain the fraction of original liquid solvent which enters with the cake and which remains on the cake exiting the final stage of the wash train, at a desired low level.
Accordingly, it is an object of this invention to control the portion of liquid solvent in the cake exiting the final stage of a multistage wash train that is the solvent entering with the cake.
It is a more specific object of this invention to compensate for uncontrolled feed disturbances entering the wash train using feedforward control techniques.
Another object is to utilize feedforward control techniques in conjunction with feedback techniques for precise control, and to rapidly handle unknown contingencies that might otherwise disturb a pure feedforward control system.
Another object of this invention is to reduce the oligomer content in the PPS product.
Yet another object is to improve efficiency in use of wash liquids.
Still another object of this invention is to develop a material balance relationship for a wash train to give results that are readily incorporated into spreadsheet type software.
Yet another objective is to recover valuable reaction additives which are soluble in a specific wash solvent.
These and other objects and aspects, as well as the advantages of this invention will be apparent to those skilled in the art upon reading the specification and the appended claims.
According to the present invention, the foregoing and other objects and advantages are attained with a steady state feedforward control system for a counter current wash operation in a polymerization process. In the counter current wash operation a polymerization reaction product, which is a solids cake saturated with an original solvent, is passed through a series of wash stages. A wash solvent is introduced to contact the cake in a counter current manner, so as to displace essentially all of the original solvent and dissolve and wash dissoluable solids before the cake exits the final wash stage. In accordance with this invention, a steady state model for feedforward control of the wash operation is developed based on a material balance, and this relationship incorporates the process load variable, the manipulated variable and the controlled variable. The feedforward control method further requires the steps of measuring the process load variable, and computing a set point for the manipulated variable as a function of the process load variable.
In a preferred embodiment of the control system, the manipulated variable is the wash solvent flow rate, which is manipulated as a function of the process load so as to maintain the weight fraction of the original solvent remaining on the cake exiting the final wash stage at a desired low level. The feedforward control scheme is enhanced by adding a feedback trimming signal. The feedback scheme comprises determining the weight fraction of the original solvent remaining on the cake exiting the final wash stage, comparing the thus determined flow to a set point, and trimming the manipulated variable set point calculated by the feedforward control system, responsive to the difference between the determined feedback variable and its set point.
The method and apparatus of this invention using a combination of feedforward and feedback control of the counter current wash operation, which handles a solvent wet mass of polymer resin, maintains the mass fraction of original solvent remaining on the exiting polymer cake at a desired low level. Further, according to this control scheme, disturbances in the process load, which can change rapidly and frequently, are detected at the input of the initial wash stage and promptly compensated by a corresponding disturbance in the manipulated variable, which will counteract the effect of the initial disturbance.
Other objects and advantages of the invention will be apparent from the foregoing brief description of the invention and from the claims as well as from the detailed description of the drawings, which are briefly described as follows: